Coastal Lagoons

Coastal lagoons are shallow, elongated surface water bodies separated from the open ocean by a partially submerged wave-deposited barrier, such as a spit, longshore bar, or barrier island. These dynamic ecosystems run parallel to the coastline and maintain structural connectivity with the open sea through one or more narrow tidal inlets. In the context of Indian geography, coastal lagoons are distinct from standard estuaries; while estuaries are river-dominated transition zones with a continuous outward flow, lagoons are wave-dominated, semi-enclosed basins where water circulation is governed primarily by tidal influx and wind-driven currents. The asymmetrical development of lagoons between India’s eastern and western maritime margins is driven by distinct tectonic and marine processes:

The Eastern Coastal Margin (Emergent and Prograding)

The East Coast of India is a low-lying, prograding emergent coast characterized by extensive deltaic sedimentation from major peninsular rivers like the Mahanadi, Godavari, Krishna, and Cauvery. Strong longshore currents transport massive volumes of littoral drift northward along the Bay of Bengal, facilitating the formation of elongated sand spits and barrier bars that trap shallow marine waters, leading to large-scale lagoon complexes.

The Western Coastal Margin (Submergent and Fault-Bound)

The West Coast is predominantly a high-energy submergent coast bounded by the structural fault line of the Western Ghats. It features narrow, rocky continental shelves, swift short-course streams, and high tidal ranges. Instead of broad lagoons, this margin features a unique network of interconnected brackish backwaters, locally classified as Kayals, formed by marine transgression and the development of natural alluvial barrier bars across river mouths.

Comprehensive Inventory of Major Indian Coastal Lagoons

The following matrix provides a holistic, 360-degree overview of the primary natural lagoon and backwater systems across the Indian littoral zone, detailing their geographic locations, hydrological properties, and ecological indices:

Distinct Morphological and Eco-Hydrological Phenomena

The Four-Sector Salinity Gradient of Chilika

Chilika Lagoon does not exhibit uniform chemical or physical parameters. It is divided into four distinct ecological zones based on salinity and hydro-dynamics: the Northern Sector, the Central Sector, the Southern Sector, and the Outer Channel Sector. During the Southwest Monsoon, heavy freshwater discharge from the Mahanadi River distributaries drops salinity levels in the northern zone to near-zero (<5 ppt). Conversely, during the dry winter and summer months, tidal ingress from the Bay of Bengal through the narrow, shifting outer channel mouth raises salinity in the central and outer sectors to near-marine levels (30 ppt). This dynamic salinity gradient regulates the lifecycle and breeding migrations of commercial catadromous and anadromous fish species.

The Thanneermukkom Engineering Divide

Vembanad Lake is bisected by the Thanneermukkom Bund, a massive saltwater barrier constructed in 1975 to protect the low-lying agricultural lands of the Kuttanad region—India’s lowest altitude farming zone, operating nearly 0.5 to 2 meters below sea level. The barrier splits the lagoon into two distinct hydrographic regimes: the southern landward zone remains a perennial freshwater reservoir fed by the Pamba and Achankovil rivers, allowing triple-crop paddy cultivation, while the northern seaward zone remains brackish, maintaining direct tidal connectivity with the Arabian Sea via the Kochi port inlet.

The Marine Buffer Matrix of Pulicat Lagoon

Pulicat Lagoon is structurally isolated from the open high-energy waves of the Bay of Bengal by the elongated barrier spit of Sriharikota Island. This geographic configuration acts as a natural buffer zone, absorbing oceanic storm surges and cyclonic energy. It also creates a highly stable, low-disturbance microclimate within the lagoon mudflats, providing forage and nesting conditions for over 150 species of shorebirds along the Central Asian Flyway.

Institutional Safeguards and Statutory Frameworks

Coastal Regulation Zone (CRZ) Notifications

Coastal lagoons are categorized under CRZ-I (Ecologically Sensitive Areas) and CRZ-IV (Water area from Low Tide Line up to twelve nautical miles seaward and water body boundaries) under the Environment (Protection) Act, 1986. These statutory rules prohibit industrial setup, untreated waste disposal, and land reclamation within lagoon perimeters, mandating that individual states draft comprehensive Integrated Coastal Zone Management Plans (ICZMP).

The National Plan for Conservation of Aquatic Eco-systems (NPCA)

Administered by the Ministry of Environment, Forest and Climate Change, the NPCA provides targeted financial and technical assistance to state wetland authorities. Across India’s coastal lagoons, this program funds core structural interventions including shoreline bio-fencing, mechanical de-siltation of choked tidal inlets, restoration of connected river channels, and community-led sustainable fisheries management.

Environmental Vulnerabilities of Lagoon Ecosystems

• Siltation and Shoaling: Deforestation and unsustainable land-use patterns in upper catchments increase the sediment load carried by feeder rivers. This heavy deposition fills up lagoon beds, causing shoaling and reducing depth and volumetric storage capacity.
• Tidal Inlet Choking: Longshore littoral drift continuously deposits sand across the mouths of lagoon inlets. If unmanaged, this process blocks the inlet, restricting seawater exchange and disrupting the lagoon’s estuarine balance.
• Eutrophication: Intense runoff of synthetic nitrogenous and phosphatic fertilizers from adjacent deltaic agricultural zones dumps nutrients into lagoons like Kolleru and Pulicat. This causes rapid algal blooms that deplete dissolved oxygen levels, leading to mass mortality of benthic fauna.
• Aquaculture Proliferation: The unauthorized construction of extensive commercial shrimp farming enclosures, locally termed gheries, physically fragments lagoon spaces, alters natural wave circulation, traps chemical effluents, and accelerates mudflat degradation.

UPSC Prelims Fact-File and Hydrological Trivia

The Strategic Isolation of Sriharikota

Pulicat Lake plays an important role in India’s space infrastructure. The barrier island of Sriharikota, which isolates the lagoon from the open sea, serves as the geographical site for the Satish Dhawan Space Centre (SHAR). The extensive, uninhabited water sheet of the surrounding lagoon provides a vital acoustic cushion and safety buffer zone for polar and geosynchronous rocket launch paths.

The Irrawaddy Dolphin Sentinel

Chilika Lagoon is the primary natural habitat for the vulnerable Irrawaddy Dolphin (Orcaella brevirostris) in India. These cetaceans are highly sensitive to hydrological changes and rely on the precise salinity variations maintained by the open tidal inlet. They serve as a key indicator species for monitoring the ecological stability of the lagoon.

The Montreux Record Rehabilitation Case Study

In 1993, Chilika Lagoon was placed on the Montreux Record—a register of Ramsar sites where ecological character has degraded due to human interference—because sandbar siltation had choked its opening to the sea, dropping salinity levels and causing a collapse of local fisheries. In 2000, the Chilika Development Authority (CDA) engineered a new artificial mouth directly linking the lagoon to the Bay of Bengal. This restored the salinity gradient, revived the ecosystem, and led to Chilika becoming the first Asian wetland to be successfully removed from the Montreux Record in 2002.

The Salt Harvesting Purity of Sambhar vs. Coastal Lagoons

Unlike coastal lagoons which receive continuous marine or riverine water changes, Sambhar Lake in Rajasthan is a true inland endorheic structural basin with no oceanic connection. While coastal lagoons yield sub-surface brackish mixtures, Sambhar relies entirely on the solar evaporation of concentrated run-off brine trapped over clay basements, yielding high-purity sodium chloride crystals used for specialized industrial salt processing.